This invention relates to a process and apparatus for gasifying coal at elevated temperatures and pressures with the aid of oxygen and one or more oxygen-free gasifying agents in a reactor, which has a rotatably mounted grate (rotary grate) for discharging ash and for feeding the gasifying agents.
Gasifying reactors of that kind consist usually of a pressure container, which is suitably surrounded by a water jacket. A pressure lock for feeding the coal to be gasified and an outlet for the raw gas product are provided at the top end of the container. A rotary grate is provided at the lower end of the reactor and cooperates with an ash discharge lock and serves also to distribute the gasifying agents into the fuel bed. Pressure reactors of that kind have been described, e.g. in German Pat. No. 828,759 and U.S. Pat. No. 2,667,409.
Such pressure reactors are operated with a substantially stationary fuel bed. The coal is charged to form a bed of high density and is progressively gasified with formation of ash as it descends toward the rotary grate whereas new material to be gasified is simultaneously strewed onto the fuel bed.
The rotary grates of such reactors must be operated under extremely severe conditions. Above the rotary grate, the gasification zone is disposed, in which a combustion reaction is performed at temperatures above 800.degree.C. The highest temperatures in the coal bed are reached on the level of the rotary grate and the temperatures decrease upwardly toward the distributor. Besides, the rotary grate is constantly in contact with the hot ash, which is discharged by the grate. The gasifying agents consisting of elementary oxygen or air or a mixture of the two, on the one hand, and steam and, if desired, carbon dioxide or nitrogen, on the other hand, flows through the interior of the rotary grate into the gasification zone.
Because of these operating conditions, the material and design of the rotary grate must be selected to meet unusually high requirements. This is particularly applicable to the design of the bearing, in which the grate is rotatably mounted, In known grates, the bearing is exposed to a high-oxygen atmosphere formed by the gasifying agent so that the lubrication of that bearing is rendered very difficult because lubricating oils quickly lose their lubricity under the influence of oxygen at temperatures above 300.degree.C. Another disadvantage of known gasifying reactors resides in that those parts thereof, which are hottest and for this reason are most highly stressed are constantly in contact with oxygen-containing gasifying agents and local extreme temperature rises may result in a combustion of the red-hot steel parts so that the grate is partly destroyed. Such damage may result in even more dangerous situations if it causes molten iron to enter a high-oxygen atmosphere so that the combustion proceeds there and results in a further temperature rise and the entire gas producer may thus be destroyed.